A circadian rhythm is a natural, roughly 24-hour oscillation in an organism's biological processes, named from Latin words meaning around and day. These rhythms are generated internally by the organism itself rather than imposed from outside, but they are reset and kept in sync with the environment by cues called zeitgebers, chiefly light, as well as temperature and other periodic signals. Circadian rhythms have been observed across animals, plants, fungi, and cyanobacteria, and evidence suggests they evolved independently in each of these groups; in humans the rhythm is coordinated by the brain's suprachiasmatic nucleus, which governs the timing of sleep, hormone release, and body temperature. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/
Facts
PeriodicityApproximately 24 hours: an endogenous cycle that repeats roughly once per day and continues even without external time cues. 1 ScaleFound from the level of individual cells up to whole organisms, across animals, plants, fungi and cyanobacteria, having evolved independently in each of these kingdoms. 1 First DescribedFirst demonstrated experimentally in 1729 by French scientist Jean-Jacques d'Ortous de Mairan, who showed that a Mimosa pudica plant's roughly 24-hour leaf movements persisted in constant darkness. 1 Identification
Distinguishing AppearanceNot a visible form: identified by a roughly 24-hour pattern in behavior or physiology, such as sleep-wake timing, that keeps running on its own even when external light-dark cues are removed, unlike a rhythm that is simply a direct response to daylight. 1 Connections
Associated With
Source Photoperiodism (Wikipedia)
Sources
1. Circadian Rhythm (Wikipedia)
Lead section, paragraph on animals, plants, fungi and cyanobacteria
Circadian rhythms have been widely observed in animals, plants, fungi and cyanobacteria and there is evidence that they evolved independently in each of these kingdoms of life.
History section, the 1729 Mairan experiment
In 1729, French scientist Jean-Jacques d'Ortous de Mairan conducted the first experiment designed to distinguish an endogenous clock from responses to daily stimuli. He noted that 24-hour patterns in the movement of the leaves of the plant Mimosa pudica persisted, even when the plants were kept in constant darkness.
Lead section, on distinguishing circadian from merely diurnal rhythms
diurnal rhythms should not be called circadian rhythms unless they can be confirmed as endogenous, and not environmental.
Quote
A circadian rhythm (), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours.
View the SourcePhotoperiodism (Wikipedia)
Associated With: Photoperiodism, Plants section, on circadian rhythm and photoreceptors sensing photoperiodQuote, Associated With: Photoperiodism, Plants section, on circadian rhythm and photoreceptors sensing photoperiod
Many flowering plants (angiosperms) use a circadian rhythm together with photoreceptor protein, such as phytochrome or cryptochrome, to sense seasonal changes in night length, or photoperiod, which they take as signals to flower.
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